This invention relates to the electronic processing of an analog signal generated in a diode array to produce a digital signal indicative of the illumination of the photodiodes in the array.
U.S. Pat. No. 3,432,670 shows a prior art diode array for sensing the illumination thereof. The array consists of ladder rungs each composed of an opposed addressing diode and a photodiode with series resistors connecting the rungs at one side. The other side of the ladder provides a terminus for the rungs.
A bias voltage is applied in series with the resistors which act as a voltage divider. In order to scan the array, a ramp voltage is applied between the resistors and the terminus. As the voltage at each rung becomes sufficient to overcome the divided bias voltage and forward bias each successive addressing diode, the respective photodiode is interrogated. Depending on the illumination of the photodiode, an amount of current flows therethrough to the terminus. This current is sensed and differentiated to provide a pulse train indicative of the illumination state of the array.
For each conducting photodiode in the array, a current flows through one or more of the series resistors. This current has the effect of changing the voltages established along the divider. This in turn can affect the identification of the photodiode being interrogated, as the respective point in time of forward bias changes. It is because of this that the bias voltage is necessary. The bias voltage needs to be high enough so that the effect of changes in interrogation times due to photodiode conduction/non-conduction do not produce ambiguous identification of the photodiode being interrogated. This requires the bias current to be substantially higher than the maximum current due to illuminated photodiodes.
U.S. Pat. No. 4,785,191 discloses diode arrays without a resistive voltage divider and is included herein in its entirety by reference. In these arrays, the addressing diodes form a series voltage dividing network. FIGS. 1-3 herein show three configurations of this type array.
In FIG. 1, addressing diodes 10 are connected in series and photodiodes 12 are connected in general to the connections or unions 14 between the addressing diodes 10. The opposite end of the photodiodes are connected to a common terminus 16. The polarity of the devices may be as shown or all inverted from what is shown.
The photodiodes 12 may be interogated by applying a voltage ramp to the terminus 16 and the terminal 17. For the diode polarities shown, the positive going side of the ramp would be applied to the terminal 17. As the threshold potential of each successive addressing diode 10 is reached a current will be able to flow through the respective photodiode 12 if it is illuminated. Thus there will be a current step for each illuminated diode and no step (or a much smaller step) for each dark diode.
Referring to FIG. 2, a similar array is shown wherein the photodiodes 12 are replaced by photoresponsive elements made up of a photodiode 18 and a non-photoresponsive diode 20. Once again, the polarities of the devices may all be inverted. In addition, the order of the diodes 18, 20 may be interchanged.
Similarly, the array of FIG. 3 has the photodiodes 12 of FIG. 1 replaced by double series photodiodes 22, 24.
The number of photodiodes or photoresponsive elements, "n", in the arrays may be, for example, sixteen.
The voltage across a resistor is a linear function of the current. In an array having a resistive voltage divider, this accounts for the relatively large change in potentials along the voltage divider in response to currents through the photodiodes. This voltage shift is such that techniques such as bias voltages and/or additional circuitry, such as means to directly sense the state of the addressing diodes, are necessary to determine the point in time when each addressing diode is forwardly biased. U.S. Pat. No. 3,448,275 shows one such approach.
In arrays having series addressing diodes which act as a series diode voltage divider as well (e.g. FIGS. 1-3), the potentials along the addressing diodes are a logarithmic function of the current. As a result, the change in potentials in response to differing photodiode illumination patterns is much smaller than in those with resistors. This permits a much simpler method and circuit to be used to determine the illumination state of the array.